A combined drawing-twisting process was developed as a new method for forming helical inner-grooved aluminum tubes. The process was applied to an extruded tube with straight inner grooves. The inner groove shape maintained that of the original tube, and no roughness or burr formation was observed. The effect of drawing reduction on the torsional buckling of the tubes was experimentally investigated. Torsional buckling can be suppressed by increasing the amount of drawing reduction. To investigate the buckling condition, the deformation in the die was analyzed by the finite element method. The stresses associated with diameter reduction and longitudinal elongation increase in the die with the amount of drawing reduction. This results in a decrease in shear stress to meet the yield condition. As a result, the torsional torque at the entry side decreases. A comparison of the experimental and analytical results shows that the experimental conditions that tend to cause torsional buckling correspond to the analytical conditions that bring the stress state at the entry side of the die close to the yield condition. The forming conditions that suppress torsional buckling can be set so that the stress state at the entry side resulting from torsional torque and backward tension does not meet the yield condition.